Spatial reference system
A spatial reference system (SRS) is a means of assigning coordinates to a location and establishing relationships between sets of such coordinates. A spatial reference system consists of the following:
- Coordinate system (i.e. x/y or lat/lon)
- Whether the SRS is geographic or projected, and
- If the SRS is geographic, the horizontal datum on which the coordinates are based, or
- If the SRS is projected, the projection on which the coordinates are based
The SRS used by most GPS units is (lat/lon, geographic, WGS84).
See http://spatialreference.org/about/ for a good overview of spatial reference systems, especially for those known to GDAL.